A material for protecting ultraviolet rays is used in sunscreens, lipsticks and foundations in the cosmetics field, and exterior walls and signboards in building materials, and paints used to vehicles and the like, etc. When used as cosmetics, since they are applied directly to the skin, transparency and safety are important in addition to ultraviolet ray protection ability. Also when used as a paint, not only color vividness and designability, but also light resistance against degradation by sunlight irradiation and durability against environmental change associated with weather changes are required. Thus, a substance that protects coating materials from ultraviolet rays and the like is used to protect components contained in a paint and a coating film, in a method of mixing it in the paint or in a method of coating it on the coating film.
Generally, use of a metal oxide as a material for protecting components in the coating material from ultraviolet rays and the like is effective for such coating materials. When the metal oxide is an iron oxide, it is required to use the iron oxide in a smallest possible amount and to reduce the influence of visible light, in order not to spoil protective ability from ultraviolet rays and the like for components in the coating material, as well as a tint generated from the coating material, its chroma, color properties such as transparency, and esthetics when applied to a human body, and designability of the coated products.
As of an iron oxide for protecting a coating material, Patent Literature 1 discloses a coloring pigment for sunlight high reflecting coating, comprising red iron oxide or yellow hydrous iron oxide having an average particle diameter of 10 nm to 300 nm. Patent Literature 2 discloses an iron oxide as a needle-shaped silica-coated Bengara red pigment having an average length of 500 nm and an average diameter of 100 nm.
However, in general, when a substance is present as nanoparticles in a dispersion, it may exhibit quite different properties which cannot be predicted from the original characteristics of the material. Patent Literature 1 and Patent Literature 2 describe properties of powders of the described iron oxide or silica-coated iron oxide regarding reflectivity and color difference, but do not describe its properties as a dispersion of nanoparticles at all. Accordingly, when a paint using a dispersion wherein the silica-coated iron oxide is nanoparticulated, is used for a coating film or a coating material, it could not be predicted whether the obtained paint would exhibit both of protection against ultraviolet rays and the liken and transparency. In addition, also when the oxide particles are used for applying them to a human body, ultraviolet ray protection ability of the iron oxide itself is important. However, Patent Literature 1 and Patent Literature 2 describe the silica coating for inhibition of photocatalytic activity in the described silica-coated iron oxide, but do not describe specific silica coating for controlling ultraviolet absorptivity at all.
Further, Patent Literature 5 discloses a metal oxide comprising an iron oxide wherein the average particle diameter of the iron oxide is in the range from 0.001 μm to 1.0 μm, and the absorbance of the metal oxide sol at the solid component concentration of 0.05 wt % at the wavelength of 800 nm is 0.1 or less, and the absorbance at the wavelength of 560 nm is 0.5 or less. Patent Literature 5 suggests improvement in dispersibility and light resistance by modifying the particle surface with silica, but each Examples does not describe a silica-coated particles. Therefore, the iron oxide particles themselves have a weak absorption ability against a light in the range from 190 nm to 380 nm, the ultraviolet region including UVA, UVB and UVC, and may not be used for a coated body and the like for which application to a human body and weather resistance are required.
In a multilayer coating film and a highly designed multilayer coating film described in Patent Literature 3 or Patent Literature 4, difference between highlight and shade for a particular color is increased, and thereby intensity of the reflected light varies greatly depending on the observation angle, to realize depth feeling and dense feeling. Therefore, for a coating film comprising a coloring material such as an iron oxide, it is required to improve the transmittance for a particular color in order to enhance the highlight, and to increase difference between highlight and shade. As a molar absorption coefficient indicating ultraviolet ray absorption ability of an iron oxide is larger, it is possible to improve transparency of the coating film as an iron oxide particle dispersion, and to make the haze value smaller by reducing its amount. However, in the case of using the conventional silica-coated iron oxide as described in Patent Literatures 1 and 2, the molar absorption coefficient for the light of the wavelengths from 190 nm to 380 nm is small, and the transmittance in the visible region tends to be lower, and the haze value tends to increase, and thus designability of the multilayer coating film product are impaired.
Patent Literature 6 filed by the present applicant discloses a method of producing various nanoparticles of an iron oxide and the like between two processing surfaces being capable of approaching to and separating from each other and rotating relative to each other. However, the described iron oxide nanoparticles are the nanoparticles of black iron oxide (Fe3O4: magnetite) and yellow iron oxide (FeOOH: goethite), and it was not observed that the iron oxide nanoparticles have ultraviolet ray protection ability, or properties to transmit a visible light and to absorb an ultraviolet light. Further, in the first place, suppression of the specific characteristics expressed in oxide particles themselves has not been described, and thus, color characteristics of oxide particles themselves have not been investigated sufficiently so far. Therefore, a composition for coating is desired which can be suitably used in both aspects of ultraviolet ray protection ability and aesthetics or designability.